This invention relates to a process for the preparation of cyclohexanone. Processes are known in which benzene is hydrogenated in a gaseous phase into cyclohexane, the cyclohexane is oxidized in a liquid phase with oxygen or a gas containing oxygen to form a mixture containing cyclohexanol, and the resulting cyclohexanol is catalytically dehydrogenated into cyclohexanone. Hydrogen gas is formed as a by-product in the dehydrogenation step, but is generally of a quality unsuitable for chemical synthesis because of the presence of organic impurities. Even after being cooled to remove the impurities to the extent possible by condensation, the hydrogen gas can still be used only as a fuel gas with a very low calorific value.
Attempts have been made in other processes to use hydrogen gas liberated during a dehydrogenation step in a hydrogenation step. For instance in British Patent Specification No. 939,613, a process for the preparation of phenol is disclosed wherein benzene is hydrogenated into cyclohexane, the cyclohexane is oxidized into a mixture of cyclohexanol and cyclohexanone, and this mixture is dehydrogenated into phenol and hydrogen. Although this hydrogen is thereafter used in the hydrogenation step, it is impossible to efficiently effect the benzene hydrogenation in a gaseous phase using this contaminated hydrogen, particularly with a metal catalyst from the platinum group, inasmuch as the catalyst very soon looses its activity. This is most probably due to the presence of organic impurities in the hydrogen gas coming from the cyclohexanol dehydrogenation step.
It is very desirable, however, to be able to effect the benzene hydrogenation in a gaseous phase, and preferably with a metal catalyst from the platinum group. Under such conditions a high rate of reaction can be obtained with a very high yield of cyclohexane, and furthermore benzene containing sulphur can readily be processed. It is therefore an object of the present invention to provide a process for the preparation of cyclohexanone wherein the contaminated hydrogen gas liberated in the dehydrogenation step can be simply and sufficiently purified for use in the gas phase benzene hydrogenation step.